As the number of users of cellular radio systems increases and rapid data transmission becomes more and more common in the systems, it becomes essentially important to increase the capacity of the system by improving the system performance.
The channel used causes frequently different kinds of interference for data transmission, for example attenuation and corruption of the signal. The received signal is often corrupted by intersymbol interference (ISI).
In receivers using TDMA (Time Division Multiple Access), an adaptive channel equalizer is often used, by means of which signal corruption and attenuation caused by the channel can be taken into account. In receivers using CDMA, (Code Division Multiple Access), the corresponding function is performed by a Rake receiver well known by those skilled in the art.
The equalizer must know the impulse response of the radio channel in order to perform successful equalization. A known way of performing channel estimation and signal equalization is to generate an estimate of the impulse response of the radio channel, in other words a channel estimate, and to equalize the received radio bursts by using the received equalization data. In a Rake receiver, each Rake finger typically comprises a channel estimator, by means of which a complex channel coefficient is estimated for each multipath-propagated signal path to be corrected.
One way to increase the capacity is the use of one or more adaptive antenna arrays instead of sector antennas. In an antenna array, single antenna elements are typically positioned close to each other, for example at a distance of half a wavelength from each other. Typically, the number of antenna elements is divisible by two to facilitate Fourier transform. An adaptive antenna array can be used in two ways: a user-specific antenna beam is turned dynamically towards the receiver, in which case the movement of the receiver is followed by an antenna beam; or the most suitable one of the antenna beams is selected, in which case a fixedly directed antenna beam is changed when the receiver is moving. When fixedly directed antenna beams are used, the basic principle is to form a set of narrow, adjacent antenna beams covering partly overlapping sectors. The most suitable one of the antenna beams is selected for the downlink channel transmission. Reuse of frequencies can be made more efficient and the power of transmitters reduced, because interference caused for other users is decreased owing to the directivity of the antenna beams.
Estimating the downlink channel is problematic in systems using an adaptive antenna array, because in such systems the user equipment can simultaneously receive several different antenna beams with control channels.